Switching power supply for neon signs

What is it you hope to do? Can we assume you plan on switching the primary, rather than the secondary? :-) If you use a 50/60Hz neon- sign transformer, you'll have to use *low* switching frequencies, and it shouldn't be too painful. On the other hand, if you want to design and/or build a 450W 15kV high-frequency current-limiting transformer, I can respectfully suggest that you don't have enough knowledge and experience. If this is really what you want to do, then start with a lower-power lower-voltage version to learn the ropes. Say 50W and 1kV. Alternately, you may be able to find a commercially-available version of a 100 to 450W 15kV smps neon-sign transformer. Let us know about it if you do.

I'd go look at a catalog of replacement TV flyback transformers for large projection TV's. Those generate a few watts at 20 to 30 KV.

They're NOT cheap, but much cheaper than the year or two it would take the average person to figure out the technology for winding 10,000 turns of #40 wire without breaking and with the requisite insulation and potting.

Then don't bother designing up a switfching power supply from scratch-- use an old PC power supply, they're available for nearly free. Just piggyback your transformer onto one of those.

While I don't mean to imply winding HV transformers is easy, not all require thousands of turns. For example, my own 10kV 300kHz 20kVA transformer has only 22 turns of #12 litz on its secondary, and only one thick 1"-wide copper-strip turn on the primary, with 450V drive. :-) It's very much a matter of the ferrite core.

I think normally the transformer secondary will be separated into sections. There are standard bobbins that have the sections with insulators molded in.

You're asking for a 450W CC supply. These things are pretty mature now and are pretty cheap to buy-- if you're trying to come up with a design for production you may wish to dissect a few commercial units to see how they have made the transformers reliable long-term, there are probably a few tricks that won't be obvious on the first pass. Corona (the products of corona) eat(s) away at thin wires (and enameled wire through pinholes) so you will likely have to coat or pot the secondary.

Best regards, Spehro Pefhany

Have you seen those piezoelectric transformers? At 5 watts each you would need a whole whack of them ($5 each), but they're very compact (1 cm x 4 cm) and quite efficient. They resonate at 53 kHz and produce max

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-- Joe Legris

There are lots of commercial switching-based "neon sign transformers" around. They seem, to me, to produce a more diffuse looking light in the tubes, less classical neon-looking. Maybe the glow is more uniform across the cross-section of the tube with high-frequency drive.

John

This one is only $40, quantity 1. It might be useful for something or other some day.

John

Wow! I'm used to puny transformers that put out about 0.2 volts per turn.

There really are cores that put out nearly twenty-five hundred times as many volts per turn?

Arent you in danger of demagnetizing most of the East Coast with that thing?

And I thought Tesla was pushing things with his lightning tower.

--- Neon sign transformers are designed to saturate when their secondary current reaches a design limit, and the impedances of their primaries are designed to limit the input current to a value which won't destroy the transformer when that happens, even if the output is feeding a short.

Basically, what they're designed to do is to provide a voltage high enough to strike an arc through a gas which presents a high impedance prior to ionization and then, once the arc has been struck and the load looks like a low impedance, to limit the current through that load.

Essentially, a constant current transformer.

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--- LOL, the primary isn't a single turn.

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--- I wasn't aware that _our_ East Coast (if that's what you're talking about) _was_ magnetized, but take a look at what it takes to make the thing run:

Even with an output of 15000 volts, a load of 0.030 amperes = 450 watts and an efficiency of about 50%, It'll be taking less than a kilowatt off of the grid, Not a great threat...

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--- You're right; he was.

-- John Fields Professional Circuit Designer

Are you sure it's 15KV? It's a pretty powerful neon signs. I am familiar with 1KV neon, not with 15KV.

Hmm.. that would indicate that neon transformers are rated in short-circuit current, rather than full-load current, so the maximum output of a 30mA 15kV supply is less than 250W.

Best regards, Spehro Pefhany

I'm not sure the modern electronic drives even use a transformer. IIRC it is a half-bridge drive of a resonant L-C to strike the arc, with the neon in parallel with C, and then steady states to something approaching more of a constant power drive.

Well said, too bad so many jumped on their swords in response.